1. Field of the Invention
The present invention relates to contact lens and ophthalmic solutions and in particular relates to methods to disinfect and clean soft and rigid gas permeable (RGP) contact lenses, effectively and safely while maintaining convenience and comfort for the contact lens wearer.
By "effectively" we mean that the levels of specified pathogenic micro-organisms as well as other contaminants such as proteins, lipids, etc., are removed or reduced by a prescribed amount within the period of time contact lenses are kept in their storage case and storage solution. This is commonly taken to be "overnight" which is estimated as 6 hours. By "safely" we mean that the prescribed reduction in pathogen and other contaminant levels is accomplished without concomitant damage to the tissues of the human eye and without deleterious alteration of the contact lens itself. By "convenience" we mean that the contact lens care solution will be such that a minimum number of steps will be required to render the contact lenses clean and disinfected and the complete compliance of the wearer to the prescribed contact lens care procedures will be more likely. By "comfort" we mean that the eyes of the wearer will be able to tolerate the direct instillation of the solution.
2. Description of Related Art
Currently available contact lenses are made of hydrogels causing them to be soft so that they can be comfortably worn. Previously, contact lenses were either hard plastic (PMMA) or RGP and required the contact lens wearing patient to adjust to the uncomfortable sensation of a foreign body in the eye. The advent of soft contact lenses has resulted in an increased adoption of contact lenses by the general population.
Contact lenses are commonly worn on a daily basis and kept in a storage case/solution during the night hours or whenever they are not being worn. During the wear and normal handling of contact lenses, microorganisms as well as biomolecules such as lipids, proteins, etc. can become adhered to the contact lenses and thus transferred to the storage case/solution. Furthermore, a tear film that contains proteins, lipids, and even microorganisms, which represent the natural flora of the ocular surface, covers the surface of the eye. Any of these components found in the tear film or on the external surface of the eye or the surrounding skin can be carried into the storage case/solution on the contact lens.
Some of the microorganisms that may be transferred from the eye or fingers to the storage case/solution may multiply therein and may later be pathogenic to the human cornea or other ocular structures. When the contact lens is returned to the eye following its overnight soaking period, it is possible for these pathogens to be applied to the surface of the eye. Although human tears contain natural anti-microbial agents, a pathogen-bearing lens in contact with the cornea of the eye can serve as a reservoir of infection that might overcome the eye's natural defenses. This is especially the case for soft contact lens as the material tends to uptake the microorganisms. The result of microbial growth--bacterial, protozoan or even fungal--can cause damage to the eye resulting in impaired vision and even blindness. Therefore, contact lenses should be daily disinfected to eliminate pathogenic organisms, usually overnight, i.e., six to eight hours, to protect the wearer's eyes from infection.
As has been stated earlier, it is also possible that other materials of biological origin can be transferred to the contact lens during wear and upon handling and transfer between the eye and the storage case/solution. These materials include cellular debris, proteins, lipids, and inorganic ions such as those of calcium and magnesium. Such materials can adsorb to the surface or become embed in the sub-surface matrix of the soft contact lens often creating persistent deposits that can cause irritation by abrasion against ocular tissues, e.g., cornea and inner surfaces of the eye lids. Furthermore, these deposits can become sufficiently severe to significantly reduce the transparency of the contact lens perhaps leading to impairment of the optical performance of the contact lens. It is therefore of interest to prevent the deposition of or to break up any aggregations of these contaminating molecules during the period of overnight immersion of the contact lenses in the storage case/solution.
Various solutions have been developed over the years to ensure that contact lenses are essentially pathogen and deposit free and can be safely and comfortably worn following overnight storage. These contact lens solutions typically include anti-microbial substances as well as cleaning (active against both lipids and proteins), wetting and other agents for the disinfection and cleaning of contact lenses during storage after wear. These solutions generally have sufficient microbicidal activity that the numbers of potentially pathogenic microorganisms are reduced to a prescribed level during the overnight soaking period.
Disinfection agents typically used for other applications such as hard surface disinfection, instrument disinfection, topical skin disinfection, etc. are not necessarily applicable to contact lens and ophthalmic solutions. The high concentration used and aggressive nature of many of these agents are unsuitable for use with contact lenses due to interaction or damage to the lens or irritation to ocular tissue. "Strong" disinfecting agents are compounds such as thimerasol, chlorhexidine, hydrogen peroxide, and benzalkonium chloride. For example, three (3%) percent hydrogen peroxide instilled directly in the eye or a lens soaked in hydrogen peroxide and applied to the eye will result in pain and severe irritation.
In the case of hydrogen peroxide, prior art answers to the problem of irritation are disclosed in U.S. Pat. Nos. 3,912,451, 4,585,488, 5,145,644 and 5,7666,931. These references show various methods and chemistries wherein the disinfecting period is followed by a neutralizing step using catalase, an enzyme that catalyzes the breakdown of hydrogen peroxide to water. This approach has found some level of acceptance among contact lens wearers. However, acceptance has remained limited, because of the multiple steps of disinfection, neutralization, and rinsing are not convenient to the wearer. More importantly, the potential exists for the neutralization and rinsing steps to be completed incorrectly (non-compliance) leading to the potential for some residual hydrogen peroxide to come in contact with the surface of the eye with the onset of severe stinging and irritation.
More recently, so-called multipurpose solutions (MPS) with chemical disinfection agents, as disclosed in U.S. Pat. Nos. 4,407,791, 4,525,346, 4,758,595, 4,820,352, 4,836,956, 5,422,073, 5,560,186, 5,593,637, and 5,756,045, have largely supplanted hydrogen peroxide systems in the marketplace because they are far more convenient than the hydrogen peroxide systems. In this case the wearer need only purchase and use a single solution leading to advantages in cost and convenience. The challenge of disinfection and cleaning without harm to the eye or the lens is particularly acute with the MPS products, however, since all of the various activities, e.g., wetting, contaminant dispersion, and disinfection, are required to co-exist in a single solution without antagonistic effects of one component on the activity of another. Furthermore, because the MPS can be instilled directly into the eye, the active anti-microbial component of these solutions must provide the required degree of pathogen reduction while being free of irritating or damaging sequelae to the surface and the anterior segment of the eye or to the contact lens itself. There is no opportunity with an MPS to neutralize or rinse away the anti-microbial agent prior to applying the contact lens to the eye.
Generally therefore the art has found it difficult to formulate these MPS solutions to satisfy the following performance criteria. The successful solution must:
1. Show anti-microbial activity to reduce the numbers of common pathogens found on contact lenses to prescribed levels; PA1 2. Show an ability to retard the deposition processes of proteins, lipids, and other materials onto or into the lens and to remove such deposits if they have formed; PA1 3. Be nonirritating to the eye without the help of rinsing and/or neutralizing solutions; PA1 4. Be free of toxic metals or compounds and sensitizing agents so that no long term allergic or toxic response is provoked; PA1 5. Not adversely accumulate within or on the lens or adversely alter the wettability or the parameters (i.e., size, shape, and optical properties) of the lens or be released in amounts toxic to the eye during lens wear; PA1 6. Show adequate shelf-life (e.g. chemical stability); PA1 7. Compatible with enzymes and other agents used in artificial tears or similar accessories to contact lens wear.
A prior art alternative to the use of chemical disinfecting agents are phenolic compounds (e.g., bioflavonoids) as disclosed in De Bruijn International Application PCT/NL97/00092 and Dutch patent NL-1002484. Natural plant derived substances; such as bioflavonoids, can be employed in contact lens care products as natural disinfecting or preserving agents. While the use of bioflavonoids is desirable because they are natural plant products, the majority of bioflavonoids are complex combinations that are difficult to obtain, reproduce and assay.
As background, the method for evaluating the effectiveness of a disinfectant generally requires measuring the ability of the agent to reduce the numbers of viable organisms during a period of time consistent with the normal period of storage of contact lenses between wearings such as six to eight hours, i.e., "overnight". This reduction of numbers of organisms is typically reported in terms of the change in the common log of the microbial population as a result of exposure to the anti-microbial agent. For example, if the agent has effected a reduction in the concentration of a particular organisms in a challenge solution from 10.sup.6 colony forming units (cfu) per milliliter (ml) to 10.sup.2 cfu/ml within six hours of exposure then the change, or "log reduction", of the organism as a result of exposure to the agent would be 4.0 (logs). In other words, the number of viable organisms have been reduced to one ten-thousandth of the original level.
In procedures for verifying the effectiveness of contact lens disinfectants generally recognized guidelines call for the use of Candida albicans (a yeast), Fusarium solani (a mold), Pseudomonas aeruginosa (a Gram-negative bacterium), Staphylococcus aureus (a Gram-positive bacerium), and Serratia marcescens (a Gram-negative bacterium). It is generally accepted in the field of ophthalmic and contact lens solutions that an effective disinfectant will cause at least a three log reduction of each of the bacterial species and a one log reduction of each of the yeast and mold within the storage time advocated for the contact lens care system, typically six to eight hours, i.e., "overnight".
These tests are most often performed by challenging the solution with a concentrated inoculum (e.g., 10.sup.5 -10.sup.6 cfu/ml) of each test organism. Over time samples are taken and plated on a growth medium to estimate the number of live organisms remaining at each time point. Of particular interest is the six-hour time point that represents the duration of typical overnight storage of soft contact lenses. It should be apparent that such a challenge represents a worst case scenario since a far greater number of microbes is added than would ever be expected on a contact lens. Further, it should also be apparent that the results of the test may be significantly influenced by other components of the solution besides the disinfectant agent.
In the case of contact lens and ophthalmic solutions various agents are added to enhance compatibility with the eye. To avoid stinging or irritation it is important that the solution possess a tonicity and pH within the physiological range, e.g., 200-350 mOsmole for tonicity and 6.5-8.5 for pH. To this end, various buffering and osmotic agents are often added. The simplest osmotic agent is sodium chloride since this is a major solute in human tears. In addition propylene glycol, lactulose, trehalose, sorbitol, mannitol or other osmotic agents may also be added to replace some or all of the sodium chloride. Also, various buffer systems such as citrate, phosphate (appropriate mixtures of Na.sub.2 HPO.sub.4, NaH.sub.2 PO.sub.4, and KH.sub.2 PO.sub.4), borate (boric acid, sodium borate, potassium tetraborate, potassium metaborate and mixtures), bicarbonate, and tromethamine and other appropriate nitrogen-containing buffers (such as ACES, BES, BICINE, BIS-Tris, BIS-Tris Propane, HEPES, HEPPS, imidazole, MES, MOPS, PIPES, TAPS, TES, Tricine) can be used to ensure a physiologic pH between about pH 6.5 and 8.5.
Various viscosity building agents such as polyethylene glycol, surfactants, polyvinylpyrrolidone, polyvinyl alcohol, carboxymethyl cellulose and similar materials may be added to adjust the "body" and "feel" of the solution. Surface active agents, such as polysorbates, polyoxyethylenes and certain phosphonates may be added to ensure proper wetting and/or cleaning. Sequestering agents such as ethylenediaminetetraacetic acid (EDTA), phosphonates, citrate, gluconate and tartarate are also common additives for preservatives of, disinfection or cleaning solutions.
To date, the significant challenge in the development of ophthalmic and contact lens solutions, particularly the MPS solutions, has been to find disinfection agents with sufficient anti-microbial activity that are not at the same time damaging to the eye or contact lens. Due to the complex requirements to keep soft, hydrogel contact lenses clean, free of pathogen microbes, and comfortable to wear without damaging or changing the lens polymer or dimensional parameters and without any harm or side effects to the human eye, only very few compounds or systems have been qualified as suitable ophthalmic or contact lens solutions. It is therefore the object of the present invention to provide a solution that combines the use of a chemical agent (BDT) and natural ingredients with the strength of harsh chemical agents such as hydrogen peroxide while avoiding irritation or damage to the eye.